A molecular genetic approach will be used to analyze the specific chemical contacts made between the bacteriophage P22 Mnt repressor protein and its symmetric 17 base pair DNA operator. We will select mutations in the mnt gene that result in a decreased affinity of the Mnt repressor for the wild-type operator (dominant negative phenotype), mutations that result in an increased affinity for the wild-type operator (hyperrepressor phenotype), and mutations that result in both a decreased affinity for the wild-type operator and an increased affinity for at least one of several symmetrically altered operators (altered specificity phenotype). To identify the amino acid residues of Mnt protein that contribute to its specificity of binding, these mutations will be defined precisely by DNA sequencing. The effects of particular mutations on the affinities of Mnt repressor for wild-type and mutant operators will be quantitated in vitro. The long-term goal of this research is an understanding of how proteins recognize specific sequences on DNA molecules. Analysis of these mnt mutations should begin to reveal the nature of the recognition code that underlies DNA/protein interactions. This type of interaction is fundamental to our understanding of the differential control of gene expression.